In recent years, there have been proposed and developed various vehicle attitude control systems interacting with vehicle brake control systems to execute vehicle attitude control by controlling four wheel-brake cylinder pressures independently of each other. Generally, the vehicle brake control system often interacts with the other vehicle systems, such as a vehicle-to-vehicle distance control system, a lane-deviation prevention (LDP) system, a traction (acceleration-slip) control system (TCS), an anti-skid braking system (ABS), a brake assist (BA) control system, a collision avoidance system, a vehicle dynamics control (VDC) system and the like. One such vehicle brake control system capable of interacting with the other vehicle systems, for example, a vehicle-to-vehicle distance control system, a traction control system, an anti-skid braking system, and a brake assist control system, has been disclosed in Japanese Patent Provisional Publication No. 2002-46588 (hereinafter is referred to as “JP2002-46588”), corresponding to U.S. Pat. No. 6,582,034 whose date of patent is Jun. 24, 2003. In the vehicular brake control system disclosed in JP2002-46588, during one cycle of the brake pressure-increasing mode (the brake pressure build-up operating mode) for vehicle-to-vehicle distance control, the distance of a host vehicle relative to the preceding vehicle is properly adjusted to a predetermined relative distance by way of the automatically controlled braking force. A basic method to automatically adjust the vehicle-to-vehicle distance is to apply almost the same magnitude of braking force to each individual road wheel, while keeping the straight-line vehicle stability. When the vehicle-to-vehicle distance control system of JP2002-46588 is in operation, motor drive control (motor rotational-speed control) is simultaneously executed in such a manner that an electric motor of a pump-and-motor assembly is driven quietly to such an extent that the driver does not take notice of automatic braking action during vehicle-to-vehicle distance adjustment. Concretely, the motor speed is properly suppressed to a low level by way of pulse-width modulated (PWM) control, thus avoiding an excessive brake pressure rise occurring owing to a system failure. More concretely, immediately after switching to the pressure build-up operating mode, an initial duty ratio of the pulse-width modulated signal is set to “100%”, and held at “100%” for a brief moment. Thereafter, the duty ratio is changed from “100%” to “20%”, and held at “20%” for a predetermined period of time. After this, as soon as the host vehicle's acceleration reaches a desired acceleration value during the pressure build-up operating mode, the duty ratio of the PWM signal is switched to “0%”. As discussed above, regarding the motor drive control (motor speed control) simultaneously executed during the pressure build-up operating mode for vehicle-to-vehicle distance control, JP2002-46588 teaches the specified PWM signal duty-ratio pattern or the predetermined combined duty-ratio pattern of a 100% duty ratio, a 20% duty ratio, and a 0% duty ratio.
On the other hand, vehicle attitude control systems must serve to positively apply a properly controlled braking force to each road wheel during cornering, so as to ensure stable cornering behavior of the vehicle by producing a controlled yawing moment. During vehicle attitude control, such as understeer or oversteer suppression control, braking forces required for respective road wheels differ from each other. Therefore, the motor of the pump-and-motor assembly of the brake control system tends to be fully rotated to supply brake fluid pressures, corresponding to the required braking forces and produced by means of a hydraulic modulator (or a hydraulic actuator), to the respective road wheels. Full rotation of the motor, activated during operation of the vehicle attitude control system, leads to a problem of increased noise. Thus, it would be desirable to optimize motor drive control, in other words, noise control for noise created by the pump-and-motor assembly during the vehicle attitude control.